The invention relates to circuits for monitoring an integrated circuit's supply voltage and delivering a signal when the level is crossed, either upwards or downwards.
For example, it may be necessary to inhibit the operation of an integrated circuit when its supply voltage crosses upper or lower limit values. The signal produced in case of the detection of a crossing of a limit is then used to stop the operation of the circuit. The reason for this may be, for example, that this excessively high or excessively low voltage might give rise to abnormal operation, and that a pure and simple interruption of the operation is preferable to abnormal operation. This problem is encountered notably in applications where the integrated circuit is used to manage confidential data or data representing a market value. This occurs for example in chip cards where this data is stored in an electrically programmable memory.
This is why it has already been proposed to incorporate a circuit to detect the crossing of a supply voltage threshold level into an integrated circuit. However, the circuits commonly used have the drawback that the average current that they consume is too great. These circuits generally use a branch in which there permanently flows a DC current. The magnitude of the consumption is in the range of 75 microamperes at 5 volts, and it would be desirable to reduce this consumption. Indeed for contactless card applications, where the available energy is low, or for cards inserted into equipment carried on board (equipment such as radiotelephones), the operating voltage is in the range of 3 volts and the consumption needs to be reduced to the minimum.
The invention proposes an innovative low-power circuit to detect a crossing of a voltage threshold level. Advantages of this circuit include:
it is very simple; PA0 the average current it consumes is very low; and PA0 it enables the detection of any crossing of a voltage threshold, even by the supply voltage itself.
According to the invention, there is proposed a circuit for the detection of the overrunning of the level of an input voltage, said circuit comprising at least one first capacitor and one second capacitor and a set of switches actuated successively and arranged so that:
in a first phase, they apply the input voltage to the terminals of the first capacitor and apply the voltage present at the terminals of the second capacitor to the gate of a detection transistor, the two capacitors being isolated from each other; PA1 in a second phase, they connect the two capacitors so that the second one is charged by the first one, the capacitors being insulated from the input and from the gate of the detection transistor, the circuit furthermore comprising a means to precharge the drain of the detection transistor during the first stage, a means to discharge the second capacitor during the first stage and a circuit to latch the logic level of the drain of the detection transistor.
In this circuit, the state of the output gets switched over in the first phase if the voltage at the terminals of the capacitor at the start of this phase (this voltage being equal to a fraction of the input voltage) crosses a determined threshold. This threshold is determined as a function of technical parameters for the construction of the circuit. These technical parameters are chiefly the threshold voltage of the transistor and the characteristics of the transistors that form the latching circuit.
The latching circuit tends to maintain the logic level for the pre-charging of the drain of the detection transistor, but the voltage threshold crossing to be detected makes the detection transistor conductive in a manner that is preponderant with respect to the action of the latching circuit, thus inverting the logic level of the drain. After switching over, the latching circuit stops tending to maintain the drain of the detection transistor at the precharging logic level.